1. Field of the Invention
The present invention generally relates to power fingerprinting and more particularly to use of power fingerprinting as a technique for improving the security and monitoring the integrity of computer processing and software used on computer-based systems.
2. Background Description
In CMOS digital circuits, with every bit transition there is a transient current drain resulting from a brief short circuit in the gates and the charge and discharge of parasitic capacitance at the outputs of the circuit. In a processor, the intensity of these transient currents, hence, the total power consumed in a specific clock cycle, is determined by the total number of bit transitions that take place in that cycle. The number of bit transitions is determined by the specific instruction sequence executed, as well as their addresses and parameters. Power fingerprinting is an integrity assessment and intrusion detection solution for critical cyber systems based on taking fine-grained measurement of a processor's power consumption and comparing them against trusted signatures (patterns that result from the specific sequence of bit transitions during execution) for anomaly detection. The basic approach behind power fingerprinting is to characterize the execution of trusted software and extract its power signatures and use them as reference to compare test traces to determine whether the same code is executing.
A power fingerprinting (PFP) monitor consists of three main elements common to all pattern recognition systems, as shown in FIG. 1: sensing 110, feature extraction 120, and detection/classification 130. Sensing involves measuring the instantaneous current drain of digital hardware, which can be accomplished using a commercial current probe and a high-performance oscilloscope. Feature extraction is a critical aspect for PFP and involves the identification of statistical and temporal properties of the power consumption that uniquely identify the execution of a given software routine. This is a challenging task that requires deep understanding of the processor's architecture and the structure of the software, but which can be facilitated by building the software itself with certain characteristics that enhance signatures and improve determinism. Ideally, a signature is extracted from every execution path in the code. In cases where this is not feasible, only a few critical sections are characterized and monitored, such as OS kernel modules and core applications.
In the general power fingerprinting approach, a sensor 110 is placed on the processor's board as close to the power pins as possible. The sensor captures the instantaneous current drain of the processor. The sensor can be a commercial current probe, a shunt resistor, or a current mirror. The signal from the sensor has to be digitized at a rate higher than the processor's main clock rate. If the processor has an internal phase-locked loop to increase the operating frequency, then this becomes the effective clock frequency. Satisfactory results have been obtained using 3.5× the effective clock frequency, but this does not represent a lower bound. Several mechanisms can be used to reduce the sampling requirements.
After the instantaneous current drain has been digitized into a power trace, different signal processing techniques are applied to extract discriminatory features from the traces. After the features have been extracted, they are passed through a supervised classifier, or detector, 130 that has been previously trained using traces 140 from trusted software. This detector ultimately makes the decision of whether the software execution corresponds to the authorized software or not. A pictographic description of the general power fingerprinting approach in the prior art is presented in FIG. 1.
The decision of whether features from a specific power trace correspond to authorized execution is performed by a carefully designed detector, which compares incoming power traces against all stored signatures 140 from authorized code. When the observed traces cannot be matched with any of the stored signatures, within a reasonable tolerance, it is determined that an intrusion has occurred. Although the difference for each feature may be small, the confidence in judging an intrusion can be very high and arbitrarily set because of the large number of features.
However, current techniques and procedures must be enhanced and improved to keep pace with technology and practices being developed and used by those seeking to overcome or defeat safeguards that rely on power fingerprinting for execution integrity of computer systems.